1. Field of the Invention
The present invention relates to a temperature-compensating circuit, particularly to a temperature-compensating circuit for compensating a detected-voltage error of a pressure sensor or the like caused by an ambient-temperature change.
2. Description of the Related Art
In the case of pressure detection using a pressure sensor, a detected voltage obtained from the pressure sensor is influenced by the ambient temperature of a place on which the sensor is disposed, and therefore an accurate detected voltage may not be obtained due to a temperature change. For example, under an environment in which an ambient temperature ta changes between −40° C. and 125° C., the detected voltage u (V) supplied from the pressure sensor is shown by the following expression.u=aP{1+α(ta−ts)}+β(ta−ts)+b  (1),where “a” is a constant, “P” is a pressure (or atmospheric pressure) detected by the pressure sensor, “α” is a span-shift temperature coefficient, “ta” is an ambient temperature (° C.) of the pressure sensor, “ts” is a reference temperature (° C.), “β” is an offset drift constant (mV/° C.), and “b” is an offset voltage (V).
For example, when assuming the span-shift temperature coefficient α as 3,000 ppm/° C. and the offset drift constant β as 1.5 mV/° C., ts is equal to 25° C., α(ta−ts) is equal to 0.3 when ta is equal to 125° C. and errors occur which arc +30% more than those obtained for an ambient temperature ta=25° C. Moreover, in this case, β(ta−ts) is equal to 0.15 V and errors occur which are +0.15 V higher than those for an ambient temperature ta=25° C. It is preferable that errors caused by these temperature changes are minimized. Therefore, various temperature-compensating methods are practically used in order to keep errors caused by α(ta−ts) or β(ta−ts) within ±(2-3%) of aP under an operating environment of −40° C.≦ta≦125° C. and obtain an output voltage proportional to u≈aP.
To cancel errors due to the above α and β via an electrical circuit, a semiconductor integrated circuit has been used so far and as downsizing of a semiconductor device is accelerated, the request for downsizing a semiconductor device can be comparatively easily achieved. FIG. 16 shows an example of the above type of circuit.
FIG. 16 is a block diagram of a temperature-compensating semiconductor integrated circuit. A semiconductor integrated circuit 200 performs temperature-compensation of an output supplied from a pressure sensor 202 for detecting a pressure P to convert the pressure P to a voltage. The semiconductor integrated circuit 200 is provided with a temperature sensor 204 and the temperature sensor 204 is set nearby the pressure sensor 202 to measure an ambient temperature ta of the pressure sensor 202. The semiconductor integrated circuit 200 is provided with DC-voltage amplifiers 206 and 208 for amplifying signal voltages supplied from the pressure sensor 202 and temperature sensor 204 up to predetermined voltages. Signal lines 210 and 211 connect detected voltages supplied from the pressure sensor 202 and temperature sensor 204 to input terminals of the DC-voltage amplifiers 206 and 208. The signal line 210 supplies the pressure detection voltage u shown by the expression (1) and the signal line 211 supplies a voltage (or current) corresponding to the ambient temperature ta of the temperature sensor 204.
Moreover, the semiconductor integrated circuit 200 has A/D converters 213 and 214. The A/D converters 213 and 214 convert analog voltages obtained from output ends of the DC-voltage amplifiers 206 and 208 to digital values. A control-signal-generating circuit 216 supplies various types of control signals to blocks and controls circuit blocks in the semiconductor integrated circuit 200. A temperature-compensating operational circuit 218 is controlled by the control-signal-generating circuit 216 to digitally process aP{1+α(ta−ts)}, β(ta−ts) and b by using digital values sent from the A/D converters 213 and 214 and values α, β, and b of temperature coefficients which are input as constants and add them each other. A D/A converter 220 receives a computation result from the temperature-compensating operational circuit 218 to convert the result to an analog voltage corresponding to the computation result. A temperature-compensated pressure-detection voltage v obtained from the following expression appears from an output terminal 222.v=aP or v=aP+C  (2),where C is a constant voltage.
Since a conventional circuit is constituted as described above, a method using the circuit is a high-accuracy, less-error, and secure method as a method for temperature compensation of a sensor-output-signal voltage u.
However, the above method has disadvantages that the circuit configuration is complex and the chip size of a semiconductor integrated circuit becomes comparatively large. When it is desired to reduce the price of the semiconductor integrated circuit 200 and a sensor-output-voltage accuracy due to a temperature change is allowed within a range of 2.0 to 2.5%, it is necessary to design the circuit which is capable of meeting the request for the price by simplifying functions of a temperature-compensating circuit and reducing the number of circuit devices.